This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The oxidative stress defenses of H. pylori are essential in allowing the bacterium to persist in its infection in the stomach, where it causes ulcers. An essential part of the bacterium's defense against oxidative stress is the methionine sulfoxide reductase repair enzyme, which can convert methionine sulfoxide to unmodified methionine. Previous studies have indicated that one of the enzymes that methionine sulfoxide reductase interacts with is catalase, another protein essential to the oxidative stress response pathways in H. pylori. In this subproject, we will oxidize catalase with a chemical oxidant such as hypochlorite, and then measure the amount of oxidation of each methionine side chain by mass spectrometry. We will then incubate the oxidized catalase with H. pylori methionine sulfoxide reductasae, including a mutant reductase with attenuated activity, and examine which methionines are repaired by the reductase.